Investment casting is extensively used in the production of nickel and cobalt base superalloy blades and vanes for gas turbine engines, particularly those requiring internal cooling holes and passages. The investment casting process produces components having precise dimensional tolerances and excellent surface finishes, both of which are required of gas turbine engine components. In investment casting, a ceramic shell mold is formed around a wax pattern with one or more ceramic cores precisely positioned within the wax pattern, occupying the position of required holes and passages in the casting to be produced. The wax pattern is then removed during a firing operation while the mold and cores remain in place, thus providing a mold cavity. Molten metal is poured into and solidified in the cavity, and the ceramic cores chemically removed such as by leaching with a hot alkali solution The use of removable ceramic cores eliminates the need for machining or drilling operations to produce the required holes and passages, which may be impossible or very difficult to perform on superalloy materials.
Ceramic cores are typically manufactured by injection molding a mixture of ceramic constituents (typically powder particles) and a binder into a green core shape, followed by a firing process to first remove the binder and then to sinter the particles to each other. Generally, chemical additives or mechanical restraints are required to maintain the green core shape and dimensions during the binder removal and sintering processes. In U.S. Pat. No. 3,234,308 to Herrmann, a core composition is disclosed which includes both an organic binder and a thermoset resin. The resin maintains the core shape during the debindering and sintering processes, with the resin burnt out as the core reaches the sintering temperature. Of course, such a composition necessitates additional formation and processing, and may leave additional residue in the core which could cause contamination of the cast metal product.
Efficient debindering is critical to successful ceramic core production. It is essential that debindering be performed at temperatures high enough to minimize the debindering period yet low enough to avoid rapid gas formation or vaporization within the core and subsequent formation of blisters on the core surface. Such constraints usually require that the debindering heat treatment be performed at temperatures lower than the optimum temperature (in terms of kinetics), which extends the debindering period and adds to the cost of producing a core.
In addition to the above mentioned concerns, cracking can occur as the core shrinks by different amounts and along different stress axes during debindering and sintering. In other words, the green cores shrink anisotropically. For example, a core may shrink to a greater degree across its width than across its length, and may shrink more along different planes across its width. The stress produced during the firing process is commonly relieved by the formation of cracks in the core. Two types of cracks have been observed in sintered cores. The most detrimental cracks are termed macrocracks; they are typically visible to the naked eye and reduce the strength of the core. Microcracks are not visible to the naked eye and appear to have little effect on the strength of the core. In general, microcracks are short range cracks confined to small unit volumes within the core. While microcracking may be tolerable and in fact desirable in some circumstances, macrocracking precludes usability of the sintered core in an investment casting process. It is not uncommon in some core producing operations to encounter macrocracks in up to 50% of the ceramic cores produced.
Cores based on amorphous silica have been used in the investment casting of various alloys. Such core systems may also contain zircon, alumina and other materials, and are used because of their stability during casting, low cost and availability of the raw materials. One limitation in the use of amorphous silica based cores is devitrification, i.e., transformation of the amorphous silica into crystalline forms of silica during the firing and casting processes; in particular, amorphous silica transforms into crystobalite, quartz and tridymite, with volume changes accompanying such transformations The volume change associated with the formation of such crystalline forms of silica produces cracks during cooling after the sintering process. Generally, excessive volume changes are avoided by a judicious choice of sintering conditions. In particular, the conditions should be chosen to achieve a crystobalite content of from about 15-30%, by volume. Such amounts of crystobalite produce a sintered core having good mechanical strength. For crystobalite contents greater than about 30% by volume, cores generally are excessively cracked and cannot be used. Therefore, either sintering must be performed at temperatures low enough to avoid the formation of excessive amounts of crystobalite for extended periods of time, or other means must be utilized to inhibit excessive crystobalite formation during core production.
Consequently, what is needed in the art is a core composition which is resistant to shrinkage and the formation of macrocracks associated therewith, and a composition which can be sintered at high temperatures for short periods of time to obtain a desired degree of devitrification.